The present invention relates to a communication device and a communication method of an underwater vehicle.
Unmanned underwater vehicles used for underwater investigation, search, and the like are broadly classified into a cable type underwater vehicle connected with a mother vessel which conveys and operates the underwater vehicle through a cable for electric power transmission or communication, and a non-cable type underwater vehicle not connected with the mother vessel. Each of the underwater vehicles is used in a scene according to its characteristic.
Out of those underwater vehicle, the non-cable type underwater vehicle is capable of moving freely since being not restricted due to a cable, and is particularly advantageous in search in deep sea and the like. Further, the non-cable type underwater vehicle is advantageous in being conveyable and operable by a small-sized mother vessel since it is not necessary to take up a cable.
However, on account of being not connected through a cable, the non-cable type underwater vehicle is naturally disadvantageous in respect of information transmission (communication) and the like, and in order to eliminate this disadvantage, a variety of techniques have been conventionally adopted.
JP-A-2001-48088 discloses a technique regarding communication between an underwater vehicle and a mother vessel. Specifically, bidirectional communication by underwater acoustic communication is performed between an underwater vehicle and a mother vessel by the use of an acoustic transducer provided in the mother vessel and an acoustic transducer provided in the underwater vehicle. Underwater information acquired by the underwater vehicle is transmitted to the mother vessel on the sea according to an acoustic wave from the mother vessel on the sea through an acoustic transducer. Further, a mission of the underwater vehicle can be changed according to contents of command of the acoustic wave from the mother vessel on the sea.
A low-frequency acoustic wave is suitable for communication of the underwater vehicle as having small attenuation in water. However, the acoustic wave has the drawback of being unusable for real-time remote control since the propagation speed of the acoustic wave is low.
In the cable type underwater vehicle, for example, it is possible to instantaneously transmit an image (projected image) having been taken in water from the underwater vehicle to the mother vessel through a cable, and on the mother vessel, it is possible to control a manipulator or the like, provided in the underwater vehicle, in real time while looking at the image so as to perform an operation.
However, in a case where a non-cable type underwater vehicle performs communication by means of an acoustic wave, an image actually taken by the underwater vehicle and an image that can be seen on the mother vessel are temporally deviated different images due to occurrence of communication delay time. Hence it is practically impossible to control the manipulator or the like while looking at the image to perform an operation.
Further, the communication by means of an acoustic wave has the drawback of being susceptible to mechanical noise made by the underwater vehicle itself. Moreover, the communication by means of an acoustic wave has the drawback of being unfavorably performed due to multi-path fading in the case of communication in a shallow sea or within a restricted space.